There is often a desire to know the current being supplied to “a user” of the current, where the user might be a factory, a distribution circuit within a factory or a dwelling, or one or more devices. It is often also highly desired to know the actual amount of energy being used by “the user” such that a power supply company can charge the user for the amount of energy that they use.
The power consumed by a device supplied with a sinusoidal voltage and drawing a sinusoidal current can be calculated fromP=V*I*Cos θ   eqn 1Where:                V is the voltage,        I is the load current and        θ is the phase angle between the voltage waveform applied to the device and the current flowing in the device. Cos θ is known as the power factor.        
As is known to the person skilled in the art, the angle θ represents the phase difference between the applied voltage waveform and the resulting current as a result of inductive or capacitive loads being fed by the power line. In a simple case both are assumed to be sinusoids at the mains frequency. In such a simple system then it is relatively easy to work in terms of phase shifts. However in reality a load, such as a motor, a switched mode supply or an inverter may have a complex current draw that includes components at multiples of the mains frequency and/or a switching frequency within the load.
Furthermore, regulatory authorities often require that consumers are fairly treated by their energy providers, and hence strict tolerance limits are imposed on the accuracy of power (watt-hour) meters. Therefore it is important that such meters maintain high levels of accuracy under all expected operating conditions. This means that the mains voltage needs to be measured, the mains current needs to be measured, and that any phase difference between the mains voltage and the mains current measurements needs to be accounted for, with sufficient accuracy such that the estimate of power consumed by a load falls within the specified levels of accuracy.
The transducers may introduce errors. For example the current transducer, such as a current transformer, may introduce an error in the size of the measured current. It may also introduce a phase error in the estimate of phase between the voltage and the current, both of which can be represented on a phasor diagram. These errors may impact adversely on the accuracy of estimates of power being consumed by a load. Similarly filters, for example filters used to reject glitches, can introduce a delay in the current and voltage processing signal paths. Furthermore although the average delay though the filters for a batch of products such as power meters may be known to reasonable accuracy on a statistical basis component to component manufacturing variation may mean that the absolute delay or indeed frequency response of any given filter is not known.
It is desirable to be able to estimate the phase error introduced by a current transducer, and/or other signal processing components within a signal processing chain involved in the measurement of voltage and current within a watt-hour meter.